The technique of joining by brazing is a joining technique that is capable of joining minute and numerous portions at once. Brazing techniques that use an aluminum alloy (including pure aluminum; likewise hereinbelow) are used in the manufacture of various heat exchangers because of the excellent lightweightness and thermal conductivity of aluminum alloys. In the brazing of an aluminum alloy, brazing is performed by wetting materials-to-be-joined with a filler material that has melted. In order for the melted filler material to wet the materials to be joined, it is necessary to break up oxide films that cover the surface of the filler material and the like. Brazing methods, vacuum-brazing methods, and the like in which flux is used are known as methods of breaking up these oxide films.
A brazing method that uses flux (commonly known as the NB method) is a method in which a fluoride-based flux is applied to the surface of a filler material, after which brazing is performed in a nitrogen-gas-atmosphere furnace; this method is the most frequently used one. However, the NB method is limited in the reduction of manufacturing costs because it requires processes for applying, drying, and cleaning the flux. In addition, because flux residue has become a problem in the coolers of electronic parts, which are being installed in hybrid vehicles, electric vehicles, and the like in recent years, there is a demand for a brazing method in which flux is not used.
The vacuum-brazing method is a technique that performs brazing by making use of the break-up of oxide films by the evaporation of Mg (magnesium) in a material during heating when brazing is being performed. In vacuum-brazing methods, although brazing can be performed without using flux, these methods tend to be subject to the effects of the vacuum level, the dew point, and the like, and therefore brazeability does not stabilize. In addition, the vacuum-brazing method has problems, such as: the vacuum equipment is extremely expensive; a large amount of electric power is needed to operate the vacuum equipment; and it is necessary to clean the furnace wall periodically.
In contrast, attempts are being made (e.g., Patent Document 1) to perform fluxless brazing using a brazing sheet of an Al—Si—Mg (aluminum-silicon-magnesium)-based filler material in an inert-gas atmosphere at atmospheric pressure. If fluxless brazing is performed at atmospheric pressure, then active evaporation of the Mg does not occur, and consequently the effect of breaking up the oxide films by the evaporation of the Mg cannot be expected. However, in this case, fine oxides are formed from the Mg within the filler material. These oxides function to fragment the dense oxide films present on the surfaces of the filler material, the materials to be joined, and the like, which makes it possible to cause the filler material to flow, even at atmospheric pressure.
Incidentally, there is a problem in that Mg contained in the filler material tends to be readily oxidized by oxygen, moisture, and the like in the atmosphere during manufacture of the brazing sheet, during heating when brazing is being performed, and the like. If a thick MgO layer is formed on the filler-material surface owing to the oxidation of the Mg, then brazeability degrades. Consequently, the sites where these brazing sheets can be used are limited; for example, they are used in brazing inside hollow structures where the surface of the filler material tends not to oxidize. In addition, when brazing is performed using these brazing sheets, it is necessary to perform, for example: a pretreatment in which oxides are removed by etching the surfaces of the filler material in advance before brazing is performed; strict control of the in-furnace environment, such as by reducing as much as possible the oxygen concentration and the dew point inside the brazing furnace (e.g., reducing the oxygen concentration to 5 ppm or less and the dew point to −60° C. or less); and the like. However, the etching treatment, such as by acid washing, and the reduction of the in-furnace oxygen concentration necessitate the introduction of new equipment, which becomes a major burden for heat exchanger manufacturers.
Accordingly, as methods of preventing the oxidation of Mg in the filler material, methods have been proposed (Patent Documents 2, 3) in which a thin film, composed of a metal having a melting point higher than that of the filler material, is provided on the filler-material surface. However, in this case, there is a problem in that, owing to the presence of the thin film, it takes more time from when the filler material melts until the filler material flows than the case in which the thin film is not present. Consequently, there is a problem in that the formation of the joint is delayed, and therefore brazing failures occur. In addition, if the thin film is clad in order to prevent the oxidation of Mg in the filler material, then materials cost greatly increases.
In addition, methods have been proposed (Patent Documents 3-6) in which Mg is added to a core and not to the filler material. However, even in these methods, there are limits to the prevention of the oxidation of Mg owing to the oxygen and the like in the atmosphere. In Patent Documents 3, 4, 6, methods are also described in which the brazed article is covered with a covering during heating when brazing is being performed in order to protect it from the atmosphere. However, in this case, it is necessary to prepare a covering that conforms to the brazed article, and to introduce new equipment. In addition, Patent Document 5 describes a method in which fluxless brazing is performed only inside a hollow body, where the effects of the oxygen concentration are small, and brazing outside of the hollow body is performed using flux. However, in this method, it is necessary to apply the flux. As described above, in the case of materials in which the only thing done is the addition of Mg to the core, satisfactory fluxless brazeability was not achieved in an atmosphere (e.g., an oxygen concentration of 15-50 ppm and a dew point of −35 to −50° C.) corresponding to the interior of a common production furnace.
To improve the brazeability of a brazing sheet in which Mg has been added to the core, methods have also been proposed (Patent Documents 7, 8) in which the elements Li (lithium), Be (beryllium), or the like are added to the filler material. However, in both methods, because Li, Be, or the like adversely oxidize during the manufacture of the brazing sheet, a pretreatment becomes necessary in which surface oxides are eliminated by etching. In addition, because readily-oxidizable elements are added to the filler material, the filler material tends to be subject to the effects of the oxygen concentration in the atmosphere, the dew point, and the like. Therefore, brazeability does not stabilize.
Other than elements such as Mg that have the effect of breaking up oxide films, Bi (bismuth) can be given as an example of an element that improves brazeability. As an element that reduces the surface tension of molten-filler material and improves brazeability, Bi has been traditionally used in vacuum-brazing methods (e.g., the Al—Si—Mg—Bi-based filler-material alloy in the JIS A4104 alloy, and the like). The effects of Bi are obtained likewise even in fluxless brazing at atmospheric pressure (Patent Document 7).
In Patent Document 7 and Patent Document 9, readily-oxidizable elements, i.e. Ca (calcium), Li, Na (sodium), Be, Y (yttrium), La (lanthanum), and Ce (cerium), are added to improve brazeability. However, because these elements form thick oxide films on the filler-material surface during the manufacture of the brazing sheet, they all require an etching treatment or the like. To obtain satisfactory fluxless brazeability without performing the etching treatment or the like in an atmosphere that corresponds to the interior of a production furnace, it is necessary either to not add these readily-oxidizable elements to the filler material or to rigorously reduce the amounts added to a level at which adverse effects on brazeability do not appear.
Furthermore, to obtain satisfactory fluxless brazeability without performing an etching treatment or the like, in which the introduction of acid-washing equipment and waste-liquid-treatment equipment are essential, it is necessary to restrict the addition of readily-oxidizable elements, i.e. Mg, Li, Be, into the filler material.
In addition, because brazing sheets for fluxless brazing use readily-oxidizable elements as the elements that replace the function of the flux, the surface oxidizes and markedly changes color after heating when brazing is performed. As a result, there is a risk that brazeability, external appearance, and the like will be impaired. In addition, because Bi, which is added to reduce the surface tension of the molten-filler material and to improve brazeability, is an element that has a higher potential than Al, the corrosion resistance of the filler material adversely decreases. As described above, it is necessary to design brazing sheets for fluxless brazing considering not only simply the improvement of brazeability but also the external appearance and the corrosion resistance after brazing.